To combat heart disease, a leading cause of death and disability in many countries, physicians require detailed data on the vasculature of the heart. In vivo, intravascular ultrasonic imaging (IVUS) offers a relatively benign method of obtaining such information. Ultrasonic imaging involves transmitting an ultrasonic acoustic wavefront pulse into a body and detecting the reflection of that pulse. Reflections occur at boundaries where acoustic impedance changes. The times at which reflections are received by a transducer correspond to the depths of these impedance boundaries. By stepping a transducer through a selected angle, one can obtain a two-dimensional (angle and depth) ultrasound image that is essentially a map of impedance boundaries. The intensity and position of these impedance boundaries can then be interpreted to characterize the condition of a vessel and its immediate environment.
The quality of the image is strongly affected by its resolution, which is in turn determined by the ultrasound wavelengths used to examine a body. Shorter wavelengths, which correspond to higher frequencies, provide higher resolution images. However, higher frequencies attenuate more rapidly, limiting their use for depth examinations. Accordingly, high frequency transducers are most appropriate for high-resolution relatively shallow imaging. For example, whereas 5-20 MHz ultrasound frequencies are useful for prenatal and peripheral vessel examinations, 30 MHz and higher are desired for intravascular examinations of cardiac vasculature.
In recent years, cardiologists have increasingly come to appreciate the diagnostic value of obtaining cross sectional images of coronary arteries by the method of IVUS. Currently there are two general types of IVUS catheter systems. First, there is the synthetic aperture approach. For example, U.S. Pat. No. 4,917,097 (Proudian et al.) and U.S. Pat. No. 5,186,177 (O'Donnell) teach how the ultrasonic beam is steered electronically from a transducer using the approach of synthetic aperture. A second type is the mechanically rotated type where the image is scanned by mechanical motion. The mechanically rotated types have three subclasses. In a first subclass, either the distal (remote from the operator) transducer or a mirror is rotated from the proximal end of the catheter by an extended drive shaft, and a proximal motor (as taught by Yock in U.S. Pat. No. 4,794,931 and U.S. Pat. No. 5,000,185). In a second subclass, the rotation is confined to the distal end, where either a miniature motor is used to rotate the transducer (U.S. Pat. No. 5,240,003 and U.S. Pat. No. 5,176,141 (Bom)) or a fluid driven turbine is used to rotate the transducer or the mirror (U.S. Pat. No. 5,271,402 (Yeung and Dias)). In a third subclass, a stationary proximal transducer is acoustically coupled to a rotating acoustic waveguide which conducts the sound to the distal end (e.g., U.S. Pat. No. 5,284,148 (Dias and Melton)).
The most prevalent type of IVUS catheter in use today is the mechanically rotated system with a planar single element transducer placed at the distal end of the catheter. A reason for this preference is the superior image quality compared with current synthetic aperture systems.
Regarding the pressure field of a planar transducer radiating into a homogenous liquid, the transition distance, N from the near-field (i.e., Fresnel region) to the far-field (i.e., Fraunhofer region) is commonly represented by EQU N=d.sup.2 /4.lambda. (1)
where d is the diameter of a circular transducer (or the width of a square transducer) and .lambda. is the wavelength of sound. FIG. 1 illustrates the transition region or focal zone (about the transition distance). In FIG. 1, an ultrasound probe 10 having a transducer 16 mounted on a rotatable shaft 14 encircled by a substantially cylindrical case (or sheath) 20 is shown. The probe is positioned inside the lumen 22 of a blood vessel 26 in the body (not shown) of a patient. The transducer 16 contains a single transducer element (although a multielement transducer can also be used). The ultrasonic field produced by the transducer has a Fresnel region A, a natural focal point (located at 30) and a Fraunhofer region B as shown in FIG. 1. The shaft can be rotated in a direction D to sweep the ultrasound in a direction of rotation C. FIGS. 2A and 2B show the transducer in portion.
As indicated by Equation (1), both the physical size and operating frequency of the transducer affect the axial location of the focal zone. Imaging of coronary arteries demands high frequency transducers, usually in the 20 to 30 MHz range, to achieve adequate axial (temporal) resolution whereby a clinician can resolve layers of the arterial wall. Physical size of IVUS catheters are continually being reduced so that they may be passed further down the coronary arterial tree or through narrower obstructions. Lower limits on catheter size are set by the ability to fabricate very small transducers and also by the fact that the transducer's electrical impedance rises and sensitivity drops with decreases in area. Currently the smallest available IVUS catheters are approximately 3.0 French in size (.about.1 mm diameter). Table 1 tabulates transition distances for various size (i.e. diameter) transducers and frequencies that can be used to image coronary arteries. The data assume the transducers to be radiating into water (v=1.5 mm/.mu.sec) which has an acoustic impedance and velocity similar to mammalian tissue.
TABLE 1 ______________________________________ Diameter of Transducer 0.4 mm 0.6 mm 0.8 mm 1.0 mm 1.2 mm ______________________________________ 10 MHz 0.27 0.60 1.07 1.67 2.40 15 MHz 0.40 0.90 1.60 2.50 3.60 20 MHz 0.53 1.20 2.13 3.33 4.80 25 MHz 0.67 1.50 2.67 4.17 6.00 30 MHz 0.80 1.80 3.20 5.00 7.20 ______________________________________
Normal coronary arteries have diameters (mean.+-.standard deviation) of 4.0.+-.0.7 mm in the left main region. They narrow to 3.4.+-.0.5 mm in the left anterior descending portion and to 3.0.+-.0.7 mm in the circumflex portion of the coronary arterial tree (MacAlpin, et al., Radiology, vol. 108, Sept. 1973, pp. 567-576). Diseased coronary arteries have narrower lumens, possibly too tight for existing IVUS catheters to pass. Assuming that the catheter is positioned in the center of the arterial lumen, then it is desired that N falls somewhere between the outer wall of the catheter (.about.0.5 mm radius, currently the smallest IVUS catheter) and the vessel wall (.about.2.0 mm radius, the largest anticipated radius of a normal coronary artery). For a chosen operating frequency, the selection of the diameter of the transducer when using a planar transducer is clearly limited.
Kondo et al., in U.S. Pat. No. 4,572,201, teach the use of an elliptically shaped transducer to "improve the resolution in the direction parallel to the axis of rotation" but do not address the effect of the "acoustic case" on the focal zone characteristics. In U.S. Pat. No. 5,291,090, Dias recognizes the distortion that the sheath has on the focus and suggests the use of an elliptically shaped transducer to correct for it. However, he does not teach how the dimensions of the transducer should be determined. Lockwood et. al. (IEEE UFFC, 1994, 41(2), pp 231-235) have produced spherically-shaped, high frequency transducers but operate them without a sheath. Their transducers are also too large for intravascular imaging in vivo. What is needed is a technique to control the focal zone of intravascular ultrasound probes having sheaths, thereby to obtain an optimal cross sectional image of the vessel under examination.